GradleFx is incompatible with gradle 6.x

[bighenry]

The following errors are returned on use following an upgrade from gradle 5.x to 6.x:

> Failed to notify project evaluation listener.
   > Removing tasks from the task container is not supported.  Disable the tasks or use replace() instead.
   > Removing tasks from the task container is not supported.  Disable the tasks or use replace() instead.
   > Removing tasks from the task container is not supported.  Disable the tasks or use replace() instead.
   > Removing tasks from the task container is not supported.  Disable the tasks or use replace() instead.
   > Removing tasks from the task container is not supported.  Disable the tasks or use replace() instead.
   > Removing tasks from the task container is not supported.  Disable the tasks or use replace() instead.
   > Removing tasks from the task container is not supported.  Disable the tasks or use replace() instead.
   > Removing tasks from the task container is not supported.  Disable the tasks or use replace() instead.
   > Removing tasks from the task container is not supported.  Disable the tasks or use replace() instead.
   > Removing tasks from the task container is not supported.  Disable the tasks or use replace() instead.
   > Removing tasks from the task container is not supported.  Disable the tasks or use replace() instead.

This appears to be caused by the task removal code in: src/main/groovy/org/gradlefx/plugins/AbstractGradleFxPlugin.groovy :

 protected Task addTask(String name, Class taskClass, Closure condition) {
        //always add tasks to make sure they are immediately on the task graph,
        //but remove them after evaluation if it turns out we don't need them
        Task task = project.tasks.create name, taskClass

        project.afterEvaluate {
            if (!condition()) project.tasks.remove task
        }

        return task
    }

[bighenry]

Opened PR to address: #260

[SlevinBE]

Thanks for the PR! I'll try to handle it in the coming week.

[najamm]

@SlevinBE will you be merging this PR and creating a release?

Thanks!